This invention relates to industrial door operators, and particularly to a hydraulic control system for the door operator which provides different speeds in opening and closing the door.
Door operators are used to open and close doors that are mounted for swinging or sliding movement between open and closed positions, or for doors that are mounted to unroll and roll-up for closing and opening door openings. The industrial doors may be used for a variety of purposes such as closing the entry ways to cold storage environments. The industrial doors, especially those used for cold storage purposes, are usually either a single panel or a by-parting sliding panel which is moved by a mechanical drive system that includes a chain loop above the doors and a reversible motor for driving the chain.
Hydraulic systems for door operators have been proposed. Examples of hydraulic door operators are found in U.S. Pat. No. 4,296,570 issued Oct. 27, 1981 to George C. Balbach, et al. and assigned to the assignee of this invention. In such hydraulic systems, a reversible hydraulic pump, driven by an electric motor, is connected by a closed loop to a reversible hydraulic motor which in turn drives the mechanical drive for the door.
The speed of operation of the door is very important. When the doors are used in a cold storage environment, they must be kept open for the shortest possible duration consistent with safety. Typically, the door will be driven open at a very rapid speed and will be closed at a second slower speed to ensure pedestrian and vehicle clearance through the door opening before closing occurs. In hydraulic door operators, this two-speed operation (i.e. one speed for opening and a second slower speed for closing) has typically been accomplished by using a two-speed electric motor to drive the hydraulic pump. The pump is driven at one speed to power the motor to drive the door to an open position, and the pump is driven at a second slower speed to power the motor to drive the door to a closed position.
Another suggested way to achieve two-speed operation in a hydraulic door operator is to insert a restricted orifice in a connection between the pump and the motor so that when fluid flow is delivered by the pump to drive the motor in a direction to close the door, the fluid flow is forced through the restricted orifice and the flow is reduced, thereby driving the motor at a slower speed. The restricted orifice is not in the flow path of fluid delivered to the door in a direction to drive the motor to open the door. Since the amount of flow delivered to the motor is governed by the flow that can be generated through the restricted orifice, a single speed electric motor can be used to power the hydraulic pump and the pump can be driven at the same speed regardless of whether the pump is driving the motor to open or close the door.
The use of a two-speed electric motor has the disadvantage of cost in that such motors are significantly more costly than single speed motors. The use of a restricted orifice to restrict flow between the pump and the motor has the disadvantage that it generates considerable heat buildup due to the back pressure that the restricted orifice creates.